1. Field of the Invention
The present invention relates to a motor provided with a plurality of magnetic pole teeth in a stator. In particular, the present invention relates to a spindle motor for use in rotating a recording disk, in which demands for reduction in size and thickness can be achieved.
2. Description of the Related Art
1. Recent Trends
In recent years, as the memory capacity density of a hard disk is increased, further miniaturization of a hard disk drive has been demanded. Furthermore, developed application to portable equipment has required for durability against a vibration or a shock from the outside. Therefore, a rigidity of a predetermined value or greater has been required for a bearing, thereby enlarging a bearing loss. In the meantime, the portable equipment need be reduced in consumption of electric power in order to prolong a continuous use period of time. For the purpose of simultaneous satisfaction of these demands, a motor driving efficiency need be enhanced.
2. Structure of Spindle Motor
As a spindle motor has been generally used a DC brushless motor. The brushless motor has included a stator, an assembly on a fixing side, onto which the stator is fixed, a rotor magnet, a rotor, to which the rotor magnet is fixed, and a bearing.
Among these members, the stator has included a plurality of magnetic pole teeth, a back yoke which magnetically connects the outer peripheries or inner circumferences of the magnetic pole teeth to each other, and a coil which is wound around each of the magnetic pole teeth. Each of the magnetic pole teeth and the back yoke has been formed by laminating a plurality of silicone steel core plates having a very high magnetic permeability. The rotor magnet has been an annular permanent magnet. The rotor magnet has been arranged with respect to the stator in such a manner as to be opposite to an inner circumferential surface in a radial direction on a side reverse to the back yoke of the magnetic pole tooth. The rotor, to which the rotor magnet is fixed, has been rotatably supported by a bearing mechanism with respect to the stator assembly, to which the stator is fixed.
3. Structure in the Prior Art
In order to reduce the thickness of a motor without any decrease in torque of the motor, Patent Literature 1 discloses a structure, in which an end on a side opposite to a rotor magnet in a stator is bent in an axial direction. In this way, an area of a magnetic pole tooth in the stator, facing to the rotor magnet in a radial direction has become greater, thereby efficiently using a magnetic flux of the rotor magnet. In other words, a torque constant has been increased. As a consequence, since an equal torque can be obtained even if a current flowing in a coil is decreased, consumption of electric force can be reduced.
4. Problems Experienced by the Prior Art
At the same time when torque is to be increased, torque variations called cogging have been liable to occur. In particular, in the case of a spindle motor for use in rotating a recording disk, the occurrence of cogging has degraded a reading accuracy by the recording disk. For example, when the torque constant is increased in the above-described manner, there has been well known the occurrence of the torque variations called cogging or an enlarged electromagnetic noise. As a result, it has been difficult to enhance the torque of the spindle motor for rotating the recording disk.